Salmonellae are facultative intracellular bacteria which cause disease in humans and animals. Infection by these organisms results in disease syndrome that include enteric (typhoid) fever and gastroenteritis. Typhoid fever is a severe systemic illness which is a major problem in travelers and the developing world. Non-typhoidal Salmonella infections are increasing in the United States and are largely associated with contaminated food. Several recent major outbreaks of S. enteritidis associated with contaminated intact shell eggs have occurred in the United States. Salmonellae infections are more severe in individuals at the extremes of age and in the immunosuppressed. This is a particular problem for individuals with AIDS, as HIV-infected persons develop severe and recurring Salmonella infections. Recurrent Salmonella bacteremia is an AIDS defining opportunistic infection. S. typhimurium interactions with epithelia are important to the organism's ability to colonize and initiate infection. These interactions can be studied in vivo in a mouse model of infection and modeled using cultured immortalized epithelial cells. This grant proposes to study these model systems. The transcriptional regulation of genes essential to entrance into epithelial cells and gastroenteritis, based upon an in vitro tissue culture model, is the main subject of this proposal. Experiments are planned to study the transcription factors termed SirA and SirC-I (Salmonella invasion regulators) that regulate factors essential to signaling eucaryotic cells to initiate cytoskeletal rearrangements that ultimately lead to organism internalization. This property is important to colonization of epithelia and is likely important to cross the intestinal mucosal barrier. In addition, evidence suggests that these properties are involved in signaling at epithelial apical surfaces to stimulate inflammatory neutrophil transmigration across an intact monolayer. This inflammation likely contributes to diarrhea. This grant proposes to further define the mechanism by which these regulators control transcription of virulence genes, and to study in molecular detail their regulation and role in bacterial virulence.